1. Field of the Invention:
The present invention relates to a light wavelength converter used for information processors, such as an optical memory disc system or a laser beam printer, and optical measuring instruments, where it is required to convert a wavelength of laser beams into a short wavelength.
2. Description of the Prior Art:
An information processor, such as an optical memory disc system or a laser beam printer, and an optical measuring instrument use laser beams emitted from a semiconductor laser device excellent in focusability and directivity. In general, a laser beam emitted from a semiconductor laser device is a near infrared beam having an oscillation wavelength of 780 nm or 830 nm.
In recent years, in order to increase the amount of information to be processed in the information processors, or to enhance the measuring accuracy in the optical measuring instruments, short wavelength laser beams are required. In the information processor, the laser beams emitted from the semiconductor laser device are condensed at a predetermined place so as to write the information or images. The wavelength of the laser beams is normally proportional to the diameter of the focusing spot, which means that the shorter the wavelength of a laser beam is, the smaller the diameter of the focusing spot becomes. The smaller the diameter of the focusing spot is, the greater amount of information in terms of the recording density is written into the optical memory disc.
In the laser beam printer, the size of images to be printed can be reduced by reducing the wavelength of the laser beam, which means that the recording density and resolution are enhanced. Moreover, if green and blue laser beams are easily obtained, a high speed and high resolution color printer can be achieved by combining a commonly used red laser beam. In optical measuring instruments, the measuring precision is enhanced by shortening the wavelength of the laser beam.
Recently, it is known that a semiconductor laser device using III-V compound semiconductor materials emits laser beams having oscillation wavelengths in the 600 nm level (for example, 680 nm), but so long as group III-V compound semiconductor materials are used, it is difficult to obtain laser beams having much shorter wavelengths. Therefore, efforts are made to develop semiconductor laser devices using ZnSe, ZnS and other group II-VI compound semiconductor materials, but at present even p-n junctions have not yet been realized. As is evident from this fact, no semiconductor laser devices capable of oscillating shortwave green and blue laser beams are available because of the unavailability of suitable materials. As a substitute, a large-scaled laser device such as an argon ion laser device and other gas lasers are used to obtain green, blue and other shortwave laser beams.
In order to solve this problem, methods for obtaining green and blue shortwave laser beams have been proposed without using large-scale gas lasers but with the wavelength of laser beams oscillated by solid-state lasers and semiconductor laser devices. One of the methods is a sum frequency generation; that is, a plurality of optical frequencies are mixed to change the wavelengths of a laser beam. A present typical example is the generation of second harmonics or third harmonics where two or three waves having the same frequency are mixed. Currently, by the second harmonic generating method, green laser beams with a wavelength of 0.53 .mu.m are generated using a YAG (yttrium aluminum garnet) laser with a wavelength of 1.06 .mu.m. Blue laser beams with a wavelength of 0.415-0.42 .mu.m are also generated by using semiconductor laser beams with a wavelength of 0.83-0.84 .mu.m.
An example of the generation of second harmonics using semiconductor laser beams with a wavelength of 0.84 .mu.m is reported in "Oyo Buturi" (meaning Applied Physics) (vol. 56, No. 12, pages 1637-1641 (1987)). According to this literature, an optical waveguide is formed on a LiNbO.sub.3 substrate by a proton-exchange method so as to generate second harmonics with an optical output of 0.4 mW at a conversion efficiency of 1% by using semiconductor laser beams having a wavelength of 0.84 .mu.m and an optical output of 40 mW. When the semiconductor laser beams are introduced into the optical waveguide, which is 2.0 .mu.m wide and 0.4 .mu.m deep, second harmonics are emanated into the substrate at an incline of approximately 16.2.degree. thereto. At this point, the second harmonics and the fundamental waves are automatically phase-matched, thereby providing no restriction on the angle between the beam and the crystal and the temperature of the crystal. However, the output light has no axially symmetrical cross-section but has a crescent-shaped cross-section. Therefore it is impossible to focus the light up to the limit of diffraction, thereby making it impossible to make use of the light.
A light wavelength converter which has solved the above-mentioned problem is described in "Resumes of the Autumn Applied Physics Academic Meeting" of 1989, at page 921. This converter is provided with a glass tube in which a crystallized non-linear material is packed so as to generate second harmonics by axially symmetrical Cerenkov radiation. This second harmonics are focused up to the limit of diffraction by the use of an axial lens. This converter has a disadvantage in that it involves a troublesome packing of non-linear material in a narrow tube, which restricts the type of the material to organic substance. In addition, since the tube has a diameter of a few microns, it is difficult to produce tubes having exactly same diameters. The crystal axis becomes fixed by the relationship between the tube and the non-linear material. This prevents the full utilization of non-linear nature of the material. It is easy to obtain organic non-linear material having a large non-linear optical constant but normally it is difficult to control the crystal axis. From a practical point of view, the known light wavelength converter has a problem.